The invention concerns an electromagnetic actuator.
An actuator comprises a control circuit, a combinational circuit and a power section. The control circuit converts a control signal into a signal suitable for activating the power section. The combinational circuit combines the output signal of the control circuit with a release signal in such a way that the output signal of the control circuit is passed on to the power section only when the release signal is available. Connected to the power section via two lines is a magnetic coil. On activation, the power section effects a current flow from an operating voltage via the magnetic coil to a reference potential. A final control element contained in the power section determines the level of the current flowing via the magnetic coil to correspond to the output signal of the control circuit passed on to the power section. The final control element may operate in a continuous or clocked mode. In the first case, the magnetic coil is constantly supplied with a voltage, the level of which determines the current flowing via the magnetic coil. In the second case, the magnetic coil is fed the operating voltage in a clocked mode, the average value of the current flowing via the magnetic coil being determined by the duty factor of the clock pulses fed to the power section. If the control voltage is in analog form, the control circuit converts the control voltage into clock pulses, the duty factor of which is determined by the level of the control voltage.
Such an actuator, in which the magnetic coil is fed the operating voltage in clocked mode, is known from the document xe2x80x9cElektrisches Verstxc3xa4rker-Modul zur Ansteuerung von direktgesteuerten Regelventilen mit elektrischer Rxc3xcckfxc3xchrungxe2x80x94Typ VT 11080, Serie 2Xxe2x80x9d [electrical amplifier module for the activation of direct-controlled control valves with electrical feedbackxe2x80x94type VT 11080, series 2X], RD 29 757/04.93 of Mannesmann Rexroth GmbH. The block diagram presented in this document shows an electromagnetic actuator for a hydraulic control valve with two magnetic coils. Each magnetic coil is connected via two electrical lines to the output of in each case one power section of the actuator. Arranged between the output of a PI controller and each power section there is in each case a control circuit. The output signal of the PI controller is fed to the control circuits as a control voltage. Each control circuit converts the control voltage, present as an analog signal, into clock signals, which activate the downstream power section in such a way that the average value of the current flowing via the magnetic coil corresponds to the level of the control voltage. In addition, the control voltages have the function of a combinational circuit. In addition to the control signal, each control circuit is fed a release signal. The control circuit combines the release signal with the clock signals in such a way that the magnetic coils are fed the operating voltage only when the release signal is simultaneously available. If the release signal is not available, the magnetic coils are not fed any operating voltage via the respectively assigned power sections. The combination of the clock pulses with the release signal is intended to ensure that the magnetic coils are supplied with current only when, in addition to a control signal other than zero, the release signal is also available. The combination of the clock signals with the release signal increases the dependability of the actuator, since the magnetic coils are supplied with current only when the release signal is available in addition to the control signal. In applications with increased safety requirements, such as hydraulically operated presses for example, this measure is not sufficient, however, since it is not possible to rule out the possibility of voltage still being fed to the magnetic coils when there is no release signal. This is the case, for example, when there is a short circuit between the operating voltage and the lines leading to the magnetic coils. Such a malfunction also occurs if an electronic switch on the operating voltage side of a power section no longer assumes the blocking state, for example after overloading. Such malfunctions can occur not only in the case of actuators with final control elements operating in a clocked mode but also in the case of actuators with final control elements operating in a continuous mode.
The invention is based on the object of providing an actuator of the type stated at the beginning which allows monitoring of the effectiveness of the combination of the output signal of the control circuit upstream of the power section with the release signal with respect to the current.
This object is achieved by the features characterized according to the invention the evaluation circuit generates a switching signal, which assumes a first value as long as current flows via the magnetic coil and assumes a second value if virtually no current flows via the magnetic coil. The formation of this signal takes place independently of the formation of a current actual-value signal provided for controlling the level of the current flowing via the magnetic coil. As long as a change of the output signal of the evaluation circuit from the first value to the second value takes place when the release signal is switched off, the combination of the control signal and release signal is effective. If, on the other hand, the output signal of the evaluation circuit assumes the first value without the release signal being available, an error has occurred.
Advantageous developments of the invention are the following. The diode between the input of the evaluation circuit and the input of the threshold switch decouples the input voltage of the threshold switch from the voltage of the line leading to the magnetic coil. The capacitor ensures that the input voltage of the threshold switch does not drop below the threshold voltage of the threshold switch during the interpulse periods in clocked activation of the final control element. The resistor ensures discharging of the capacitor after the ending of the activation of the power section. If the evaluation circuit is fed the voltages of the two lines leading to the magnetic coil, the smoothing effort for the input voltage of the threshold switch is reduced. Furthermore, the zero transition of the current flowing via the magnetic coil can be sensed more exactly. If the decay time constant of the input voltage of the threshold switch is greater than the decay time constant of the current flowing via the magnetic coil, the voltage fed to the threshold switch falls below the threshold voltage of the threshold switch only when the current flowing via the magnetic coil has become zero. Since, in the conducting-state phase of the current flowing via the magnetic coil, current also flows via the resistor of the evaluation circuit, a high-impedance resistance in comparison with the resistance of the magnetic coil reduces the current requirement of the actuator. If the evaluation circuit is provided with further inputs, it is possible in the case of actuators with two magnetic coils to monitor the voltages of the lines leading to the magnetic coils with only one evaluation circuit.